This invention is directed to joule melters for glass and, in particular, to joule melters in which radioactive wastes are mixed with the molten glass for disposal.
Among the many approaches to the immobilization of radioactive fission products and actinides derived from the reprocessing of nuclear fuels, one promising technique consists of incorporating them into a durable glass. A Liquid-Fed Ceramic Melter technique is described by J. L. Buelt and C. C. Chapman in a report prepared for the United States Department of Energy in October 1978--document number PNL-2735, UC-70. In this report, as well as in U.S. Pat. No. 4,139,360 which issued on Feb. 13, 1979 with Helmut Piper as inventor, joule melters used in the immobilization of radioactive waste are described.
Molten glass is a conductor of electricity and, as the temperature of the glass rises, its electrical resistance decreases. A joule melter is designed to pass an electric current through the glass, producing heat according to the square of the current and the first power of the resistance. In the form developed for solidification of reprocessing wastes, a joule melter consists of a rectangular or polygonal box, with an inner surface of a corrosion-resistant refractory, an intermediate layer of heat-insulating refractory, and an outer metal containment. Current is passed through the glass from two or more electrodes placed on or in opposite faces of the box. Molten glass is withdrawn from the melter by means of an outlet spout, while fresh, unmelted material is introduced on the top of the melt. A usual refinement is a baffle, also of corrosion-resistant refractory, to prevent unmelted material from moving directly to the outlet.
Because it is possible to pass an appreciable current through a glass only when it is molten, it is necessary to provide startup heaters to raise the glass to a sufficient temperature for the joule-heating effect to be used. The startup heaters are normally radiant heaters installed in the space above the melt. Auxiliary heaters are often placed at the outlet to ensure a proper glass flow from the melter.
In addition to the above joule melters which have been designed as part of a radioactive waste disposal technique, many other types of glass furnaces have been used for making an purifying glass or ceramic products. For instance, U.S. Pat. No. 2,268,546 which issued on Jan. 6, 1942, is an example of a fuel fired furnace in which molten glass leaves the melting chamber through a central overflow tube. U.S. Pat. No. 3,147,328 which issued on Sept. 1, 1964, U.S. Pat. No. 3,519,412 which issued on July 7, 1970, U.S. Pat. No. 3,876,817 which issued on Apr. 8, 1975, and U.S. Pat. No. 3,912,488 which issued on Oct. 14, 1975, on the other hand, are all examples of joule melters with their molten glass outlet at the center of the melting chamber, but which have quite different electrode arrangements. Finally, U.S. Pat. No. 2,781,411 which issued on Feb. 12, 1957, describes a melter with concentric electrodes to be used for purifying glass.
Though the process of joule melting has been applied to the manufacture of radioactive glasses with some success, it has been found that in many cases the prime limitation on the working life of the joule melter has been due to the corrosion of the refractory. Though the life of a melter may be extended somewhat, the problem of disposing of an apparatus which itself becomes radioactive with time, is inevitable. The dismantling and disposal of all of the prior art devices have been found to be very difficult since they are complex and bulky.